Ingredient #1: Baking Soda
$5.24 for 13-1/2 pounds, at Costco
http://costcocorner.com/2013/05/arm-...13-5-lbs-5-24/

Ingredient #2: Cream of Tartar (or equivalent acid salt)
$6.81 for 1 pound, at a restaurant supply house
http://www.webstaurantstore.com/4504...of-tartar.html

Ingredient #3: Corn Starch
$2 for 1 pound almost anywhere
http://www.amazon.com/Barry-Farm-Cor.../dp/B00015UC5C

So, for less than $15, the OP can make baking powder, limited
only by the shelf life of the separated ingredients.

I know the baking soda lasts forever (kept dry).
So should the cream of tartar (kept dry).

How long is the shelf life of the corn starch?